Advanced upgrading of pyrolysis oil via liquid-liquid extraction and catalytically upgrading
Biomass is one of the important renewable energy sources which are used to produce carbon-based liquid fuels through biochemical and thermochemical methods. The bio–oil from the biomass pyrolysis is an intermediate and requires upgrading. Catalyst coking is the most difficult challenge during the upgrading process, and results in low hydrocarbon yield and short catalyst life time. Compared with other methods for reducing catalyst coking such as high temperature baking and co-feeding hydrogen, a new route by liquid–liquid extraction to separate chemical compounds in mixture has been established in this study. First of all, the liquid-liquid extraction process using chloroform solvent on raw pyrolysis oil successfully separated chemical compounds by their properties and polarities with high extracting yield and admirable effect on chemical distribution. The acid, aldehyde and unpyrolyzed sugar can be completely removed from the bio–oil mixture. Secondly, an esterification process over the acid-rich water phase of pyrolysis oil achieved acid conversion over 89% of by completely randomized design with temperature and catalyst loading factors. The esterification process followed the Langmuir–Hinshelwood mechanism and reaction constants were calculated to build a practical esterification model for acid compounds. Additionally, dimethyl acetal was also produced by acetalization on the water phase via acetalization. Products determined by HPLC and GC−MS represented similar Langmuir–Hinshelwood mechanism kinetics to esterification, while moisture and acid content in the feedstock are the main inhibitors for the process. Finally, raw bio–oil and liquid–liquid extracted bio–oil on ZSM–5 catalytic cracking was compared to illustrate our new pathway to solve catalyst coking. Temperature played an important role on increasing gaseous product and hydrocarbon yield as shown by GC−MS and Micro−GC analysis. Less catalyst coke and longer catalyst lives were achieved on liquid–liquid extracted bio–oil than raw bio–oil, indicating significant positive results on reducing catalyst coking. Besides, the new method developed in our study had several advantages over the traditional methods to solve catalyst coking. Neither alumina loss happened, nor high cost hydrogen was required, and other types of bio–oil also can be applied in this system.